1. Field of the Invention
This invention relates to a jack and particularly to a pneumatic jack for electric welding of the kind which comprises a cylinder housing a piston having a piston rod provided with a nose formed by an extension outside the cylinder and adapted to receive one end of an electrode carrier, the other end of which is intended to receive a welding electrode.
2. Description of the Prior Art
Jacks of the above mentioned kind, which are disposed on a frame, are more particularly intended for effecting the spot welding of metal sheets movable along a predetermined path.
Experience has shown that after the welding the electrode may remain adhering to the workpiece because the rising movement of the jack is intentionally limited.
In arrangements known at the present time no means are provided for verifying that the piston rod of the jack and consequently the electrode are actually in the top position before movement of the workpiece from one position to the next is initiated.
It follows that if this movement takes place while an electrode is still adhering to the workpiece, the movement gives rise to damage to the piston rod, and the jack must then be taken out of service.
If this should occur, it is indispensable that the jack should be replaced, thus giving rise to serious disturbances in the production cycle.
It is an object of the invention to overcome these disadvantages and to this end provides a jack with a monitoring device which is particularly simple to use and extremely effective.